Does anyone know wheres the best place i can get a middle silencer as my exhaust system is a cat back (straight pipe from cat to back boxes) so its really loud and the fact i got two back boxes makes it worse

Ive seen some in motor factors. You cut the pipe and fit them in with clamps. Try Motorist Discount Centre if you have one nearby.

the exhaust company i got my exhaust system fitted said theyll do it for £80,

I just wondered, are there any other benefits of a middle silencer apart from reducing sound? Or would i lose any performance if i did get this done?

£80 is expensive round croydon i got mine done for 45 pounds try these guys mite have a branch near you, http://www.longlife.co.uk/